Interconnect for fuel cell stack

ABSTRACT

Various embodiments include fuel cell interconnects having a fuel distribution portion having an inlet opening, a fuel collection portion having an outlet opening, and a primary fuel flow field containing channels, wherein the fuel distribution portion comprises at least one raised feature defining a fuel distribution flow path, and the fuel distribution flow path is not continuous with the channels in the primary fuel flow field. The at least one raised feature may include, for example, a network of ribs and/or dots. Further embodiments include interconnects having a fuel distribution portion with a variable surface depth to provide variable flow restriction and/or a plenum with variable surface depth and raised a raised relief feature on the cathode side, and/or varying flow channel depths and/or rib heights adjacent a fuel hole.

RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalApplication No. 61/694,337, filed Aug. 29, 2012, and to U.S. ProvisionalApplication No. 61/723,992, filed Nov. 8, 2012, the entire contents ofwhich are incorporated herein by reference.

BACKGROUND

In a high temperature fuel cell system, such as a solid oxide fuel cell(SOFC) system, an oxidizing flow is passed through the cathode side ofthe fuel cell while a fuel flow is passed through the anode side of thefuel cell. The oxidizing flow is typically air, while the fuel flow canbe a hydrocarbon fuel, such as methane, natural gas, pentane, ethanol,or methanol. The fuel cell, operating at a typical temperature between750° C. and 950° C., enables the transport of negatively charged oxygenions from the cathode flow stream to the anode flow stream, where theion combines with either free hydrogen or hydrogen in a hydrocarbonmolecule to form water vapor and/or with carbon monoxide to form carbondioxide. The excess electrons from the negatively charged ion are routedback to the cathode side of the fuel cell through an electrical circuitcompleted between anode and cathode, resulting in an electrical currentflow through the circuit.

In order to optimize the operation of SOFCs, the oxidizing and fuelflows should be precisely regulated. Therefore, the flow regulatingstructures, such as interconnects in the fuel cell system should beprecisely manufactured. One type of interconnect currently used is ametal interconnect formed by a powder metallurgy technique. The metalinterconnect is typically a chromium-based alloy.

SOFC interconnects (ICs) and endplates require low permeability to keepthe air and fuel from mixing, and good topography uniformity (flatness)to ensure that the electrolyte does not experience stress concentrationsbeyond the breaking strength of the electrolyte. The nature of PowderMetallurgy (PM) manufacturing inherently produces density non-uniformityon parts that have thickness and topography variation in the directionof compaction tonnage. Several compaction methods are already in placeto correct for density non-uniformity such as variations in powder filland compensation on tooling design, however these methods would alsocause undesirable topography on the parts.

PM technology creates the shape of a part using three components in acompaction press—the upper punch, the lower punch and a die. The designof the IC necessitates various cross sectional thickness to be molded byfeatures on the punches, i.e., there is cross sectional thicknessvariation in the direction of compaction tonnage (FIGS. 8A and 8B). Thisis different from most parts that are processed using PM technologywhere the punches are typically flat and the die is the component thatcontains the geometric features, i.e., the cross sectional thickness inthe direction of compaction tonnage is uniform (FIGS. 9A and 9B).

As a result of the orientation of the ICs in the compaction process, itis challenging to achieve density uniformity. There are severaldrawbacks to non-uniform density in compaction. Some drawbacks includecertain regions on the IC getting over-densified, therefore increasingtool wear and decreasing tool life; regions that are over-densifiedundergoing flaking or delamination; and regions with lower densityhaving higher permeability, adversely affecting functionality of ICs inthe fuel cell system. There are several standard methods used in the PMindustry to achieve density uniformity. However, this is at the cost ofhigh topography variation that causes high stresses on the fuel cell.

SUMMARY

Embodiments include an interconnect for a fuel cell stack that includesa first side comprising a fuel distribution portion, a fuel collectionportion and a first plurality of ribs and channels defining a primaryfuel flow field between the fuel distribution portion and the fuelcollection portion, a fuel inlet opening in the fuel distributionportion and a fuel outlet opening in the fuel collection portion,wherein the fuel distribution portion comprises at least one raisedfeature defining a fuel distribution flow path, and the fueldistribution flow path is not continuous with the channels in theprimary fuel flow field.

The at least one raised feature may include, for example, a network ofribs and/or dots that supports and increases contact area with a fuelcell electrolyte, distributes the fuel to provide more efficient fuelutilization, and enhances the topology and density characteristics ofthe interconnect.

Further embodiments include an interconnect for a fuel cell stack thatincludes a first side comprising a fuel distribution portion, a fuelcollection portion and a first plurality of ribs and channels defining aprimary fuel flow field between the fuel distribution portion and thefuel collection portion, a fuel inlet opening in the fuel distributionportion and a fuel outlet opening in the fuel collection portion,wherein at least one of (i) the first side comprises a plenum, and asecond side of the interconnect opposite the first side comprises araised relief that coincides with the location of substantially theentire plenum, (ii) the first side comprises at least one plenum thatextends at least about 60% around the circumference of at least one ofthe fuel inlet opening and the fuel outlet opening and a flow channeldepth in the primary fuel flow field on the first side of theinterconnect is greater adjacent to at least one of the fuel inletopening and the fuel outlet opening than distal from the at least one ofthe fuel inlet opening and the fuel outlet opening, and (iii) the firstside comprises at least one plenum that extends at least about 60%around the circumference of at least one of the fuel inlet opening andthe fuel outlet opening and a height of ribs on at least one of thefirst side and the second side of the interconnect in a portion adjacentto at least one of the fuel inlet opening and the fuel outlet opening isless than the height of the same ribs distal from the at least one ofthe fuel inlet opening and the fuel outlet opening.

Further embodiments include a method of operating a fuel cell stack thatincludes flowing fuel from a fuel inlet riser opening along a fueldistribution path defined by at least one raised feature, wherein thedirection of the fuel distribution path deviates from the direction of aline between the fuel inlet riser opening and a fuel outlet riseropening by more than 30°, flowing the fuel from the a fuel distributionpath through a plurality of parallel channels along a direction that issubstantially parallel to the line between the fuel inlet riser openingand the fuel outlet riser opening, wherein the fuel distribution path isnot continuous with the parallel channels, and flowing the fuel from theplurality of parallel channels along a fuel collection path defined byat least one raised feature and out the fuel outlet riser opening,wherein the direction of the fuel collection path deviates from thedirection of the line between the fuel inlet riser opening and the fueloutlet riser opening by more than 30°.

Further embodiments include fuel cell stacks incorporating aninterconnect as described above, as well as methods of fabricatinginterconnects for a fuel cell stack.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutepart of this specification, illustrate example embodiments of theinvention, and together with the general description given above and thedetailed description given below, serve to explain the features of theinvention.

FIG. 1 illustrates a side cross-sectional view of a SOFC stack.

FIG. 2A is a perspective view of a cathode side of an interconnect ofthe prior art.

FIG. 2B is a perspective view of an anode side of an interconnect of theprior art.

FIG. 3A is a schematic plan illustration of the anode-side of aninterconnect according to one embodiment.

FIG. 3B is a cross-sectional illustration of the interconnect of FIG.3A, viewed along line A-A of FIG. 3A.

FIG. 3C is a cross-sectional illustration of the interconnect of FIG.3A, viewed along line B-B of FIG. 3A.

FIG. 4 illustrates a portion of an interconnect having raised dots overa fuel distribution portion.

FIG. 5 illustrates a portion of an interconnect having raised dots andribs over a fuel distribution portion.

FIG. 6A illustrates a portion of an interconnect having varying depthsover a fuel distribution portion.

FIG. 6B is a cross-section view of the interconnect of FIG. 6A alongline C-C.

FIG. 7A illustrates a portion of the anode-side of an interconnecthaving a fuel plenum with a depressed high-density region surroundingthe fuel opening.

FIG. 7B illustrates a portion of the cathode-side of the interconnect ofFIG. 7A having a relief that corresponds with the fuel plenum on theopposite side of the interconnect.

FIGS. 8A and 8B are respective side cross sectional and top views of aprior art powder metallurgy (PM) apparatus for making interconnects.

FIGS. 9A and 9B are respective side cross sectional and top views of aprior art PM apparatus.

FIG. 10A is a partial perspective view of a fuel side of an interconnectin which the fuel flow channel depth is greater adjacent to the fuelinlet and/or outlet riser opening(s) than distal from the fuel riseropening.

FIG. 10B is a partial cross-sectional view of an interconnect takenalong line D-D in FIG. 10A.

FIGS. 11A-B are partial perspective views of a cathode side of aninterconnect in which the height of ribs in a portion adjacent to theriser channel opening is less than the height of the ribs distal fromthe riser channel opening.

FIG. 11C is a partial cross-sectional view of an interconnect takenalong line E-E in FIG. 11A.

DETAILED DESCRIPTION

The various embodiments will be described in detail with reference tothe accompanying drawings. The drawings are not necessarily to scale,and are intended to illustrate various features of the invention.Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts. References made toparticular examples and implementations are for illustrative purposes,and are not intended to limit the scope of the invention or the claims.

Various embodiments include interconnects for a fuel cell stack, fuelcell stacks and systems incorporating such interconnects, and methods offabricating interconnects for a fuel cell stack.

For solid oxide fuel cell stacks, the currently-used interconnects aretypically made from an electrically conductive metal material, and maycomprise a chromium alloy, such as a Cr—Fe alloy. The interconnects aretypically fabricated using a powder metallurgy technique that includespressing and sintering a Cr—Fe powder, which may be a mixture of Cr andFe powders, to form a Cr—Fe interconnect in a desired size and shape(e.g., a “net shape” or “near net shape” process). A typicalchromium-alloy interconnect comprises more than about 90% chromium byweight, such as about 94-96% (e.g., 95%) chromium by weight. Theinterconnect may also contain less than about 10% iron by weight, suchas about 4-6% (e.g., 5%) iron by weight, may contain less than about 2%by weight, such as about zero to 1% by weight, of other materials, suchas yttrium or yttria, as well as residual or unavoidable impurities.

An example of a solid oxide fuel cell (SOFC) stack is illustrated inFIG. 1. Each SOFC 1 comprises a cathode electrode 7, a solid oxideelectrolyte 5, and an anode electrode 3. Fuel cell stacks are frequentlybuilt from a multiplicity of SOFC's 1 in the form of planar elements,tubes, or other geometries. Fuel and air has to be provided to theelectrochemically active surface, which can be large.

The gas flow separator 9 (referred to as a gas flow separator plate whenpart of a planar stack), containing gas flow passages or channels 8between ribs 10, separates the individual cells in the stack.Frequently, the gas flow separator plate 9 is also used as aninterconnect which electrically connects the anode or fuel electrode 3of one cell to the cathode or air electrode 7 of the adjacent cell. Inthis case, the gas flow separator plate which functions as interconnectis made of or contains electrically conductive material. Theinterconnect/gas flow separator 9 separates fuel, such as a hydrocarbonfuel, flowing to the fuel electrode (i.e. anode 3) of one cell in thestack from oxidant, such as air, flowing to the air electrode (i.e.cathode 7) of an adjacent cell in the stack. At either end of the stack,there may be an air end plate or fuel end plate (not shown) forproviding air or fuel, respectively, to the end electrode. FIG. 1 showsthat the lower SOFC 1 is located between two interconnects 9.

FIGS. 2A and 2B show, respectively, top and bottom views of a prior artinterconnect 9. The portions of interconnect 9 shown in sidecross-section in FIG. 1 are provided along lines A-A in FIGS. 2A and 2B.The interconnect 9 contains gas flow passages or channels 8 between ribs10. The interconnect 9 in this embodiment includes at least one riserchannel 16 a for providing fuel to the anode-side of the SOFC 1, asillustrated by arrow 29. The riser channel 16 a generally comprises afuel inlet riser opening or hole that extends through at least one layerof the fuel cells and interconnects in the stack. As illustrated in FIG.2B, the fuel can flow through the inlet riser channel 16 a to theanode-side of each fuel cell. There, the fuel can collect in an inletplenum 17 a (e.g., a groove in the interconnect's surface), then flowover the fuel cell anode 3 through gas flow channels 8A formed in theinterconnect 9 to an outlet plenum 17 b and then exit through a separateoutlet riser channel 16 b. It should be noted that the ribs 10 may bepositioned in any configuration such that the air and fuel can flow onopposite sides of the interconnect 9 in the same direction (co-flow) orin opposite directions (counter-flow) or at right angles to each other(cross flow) or at any angle in between.

The cathode side, illustrated in FIG. 2A, can include gas flow passagesor channels 8C between ribs 10 which direct air flow 44 over the cathodeelectrode of the fuel cell. Seals 15 a, 15 b can seal the respectiverisers 16 a, 16 b on the cathode-side of the interconnect and fuel cellto prevent fuel from reaching the cathode electrode of the fuel cell.The seals may have a circular or hollow cylinder shape as shown so thatthe risers 16 a, 16 b extend through the hollow middle part of therespective seals 15 a, 15 b. The seals 15 a, 15 b can include anelevated top surface for contacting against the flat surface of theadjacent SOFC 1. A peripheral seal 15 c can seal the anode-sides of theinterconnect and fuel cell to prevent air from reaching the anodeelectrode of the fuel cell, and strip seals 15 d can seal the edges onthe cathode sides.

In FIGS. 2A and 2B, the riser channel openings 16 a, 16 b are shown asfuel inlet and fuel outlet openings in the interconnect 9. Thisinterconnect is configured for a fuel cell stack which is internallymanifolded for fuel, in which the fuel travels through the stack throughfuel riser channels which are formed by mated openings through thestacked interconnects and fuel cells. Furthermore, the interconnect 9shown in FIGS. 2A and 2B is configured for a stack which is externallymanifolded for air. However, additional openings through theinterconnect may be formed, such as on the left and right sides of theinterconnect, for the interconnect to be configured for a stack which isinternally manifolded for air.

The functions of an interconnect 9, such as the interconnect 9 shown inFIGS. 1-2B may include, without limitation, a) delivering the anode andcathode reactants uniformly to the active areas of the electrolyte, b)preventing the mixture of the anode and cathode reactants, c) providingstructural support of the electrolyte, and d) conducting electricity. Toachieve these functions, the interconnect 9 preferably includes fuel andair flowfields that efficiently distribute the reactants over the entirefuel-side and air-side surfaces of the interconnect. The interconnect 9preferably also possesses low permeability, and provides uniform contactto the active area(s) of the adjacent fuel cells. For interconnectsformed by powder metallurgy as discussed above, the uniformity ofcontact with the adjacent fuel cell is a function of the topographyvariation throughout the interconnect, and the permeability of theinterconnect is a function of the density of the pressed powderinterconnect. When powdered metal is compacted with features commonlyused in fuel cell interconnect designs, variations in powder density andtopography may be introduced due to varying sectional areas found in thedesign. It is also desirable to achieve the highest densities in regionsmost sensitive to crossover leaks.

The interconnect 9 illustrated in FIGS. 2A and 2B includes an inlet fuelplenum 17 a that very efficiently distributes the fuel from the fuelinlet hole out to the flowfield, and a fuel outlet plenum 17 b thatcollects the excess fuel and reactant products at the outlet anddelivers them to the fuel outlet hole. However, these plenums 17 a, 17 bare relatively wide regions resulting in a reduction of the contactbetween the interconnect 9 and the adjacent fuel cell by a significantamount. The plenums 17 a, 17 b are also relatively deep depressionsthat, due to their relatively small cross-sectional thicknesses, therebyresult in the highest density regions of the compressed powderinterconnect. The compaction process tends to “bottom-out” when thesehigh-density regions approach 100% dense. The stroke of the compactionpress is limited due to these high-density regions being incapable ofbeing compacted further. As a result, the density achieved elsewhere inthe part may be less than optimal. This density variation often leads totopography variation, which can reduce the amount of contact theinterconnect makes with the adjacent cell.

Another important consideration is in the area of operationalefficiency. Maximizing the fuel utilization is important to operationalefficiency. Fuel utilization is the ratio of how much fuel is consumedduring operation relative to how much is delivered to the cell. Fuelcell operation necessitates avoiding fuel starvation in any region ofthe active area, which drives the requirement of having appropriatelydistributed fuel within the flowfield. If there is a maldistribution offuel such that some flowfield channels receive insufficient fuel tosupport the fuel cell reaction, this will result in fuel starvation inthese channels, and possible irreversible damage to the fuel cell.Therefore, the safe way to avoid fuel starvation is to deliver an excessof fuel, thus hurting operational efficiency. As shown in FIG. 2B, thefuel-side of the interconnect 9 in this example includes parallelchannels of equal length. Optimizing fuel utilization in such a designstrives for all the channels to run out of fuel simultaneously at theend of the channel. With variations of fuel distribution and currentwithin the active area, this is rarely, if ever, the case.

FIGS. 3A-3C illustrate an interconnect 9 having a fuel distributionnetwork that includes a plurality of raised features (e.g., protrusions)according to one embodiment. FIG. 3A is a plan view of the anode (fuel)side 302 of the interconnect 9. The interconnect 9 in this embodiment isa planar interconnect having a first edge 301 and a second edge 303. Afuel distribution portion 305 is proximate the first edge 301 and a fuelcollection portion 307 is proximate the second edge 303. A middleportion 309 is located between the fuel distribution portion 305 and thefuel collection portion 307. The middle portion 309 includes a primaryfuel flow field 311, which may include a plurality of ribs 310 definingfuel flow channels 308 between the ribs 310. The plurality of fuel flowchannels 308 may be parallel to one another and configured to conveyfuel over the anode side 302 of the interconnect 9, where the fuel maycontact the anode electrode of the adjacent fuel cell in a fuel cellstack. Fuel in the primary flow field 311 may flow in a first direction,indicated by arrows 312, which may be along a substantially straightpath between the fuel distribution portion 305 and the fuel collectionportion 307. Thus, ribs 310 and channels 308 are preferably straightline shaped. A peripheral seal 315 c may seal the anode-side 302 of theinterconnect 9 and the adjacent fuel cell to prevent air from reachingthe anode electrode of the fuel cell.

The fuel distribution portion 305 includes at least one inlet riserchannel opening 316 a for providing fuel to the anode-side 302 of theinterconnect 9. In the embodiment of FIG. 3A, a raised feature 313(i.e., blocking rib) is located between the inlet riser channel opening316 a and the entrance to the primary flow field 311. The blocking rib313 prevents fuel from flowing directly (i.e., in a straight line path)from the inlet riser channel opening 316 a into the primary flow field311 of the interconnect 9. In various embodiments, the blocking rib 313is configured such that fuel from the inlet riser channel opening 316 amust flow in a second direction, indicated by arrows 314, beforeentering the primary flow field 311, where the second direction deviatesby at least 30° (e.g., 45-180°), such as at least 60° (e.g., 90°) fromthe first direction of fluid flow within the primary flow field 311.

In various embodiments, such as shown in FIG. 3A, the fuel distributionportion 305 of the interconnect 9 may include a fuel distribution flowfield 319. The fuel distribution flow field 319 may include, in additionto the blocking rib 313, additional raised features (e.g., protrusions)323, such as ribs, dots, etc., that define flow paths 325 between thefeatures 323. In the embodiment of FIGS. 3A-3C, the raised features 323comprise a network of ribs. The raised features 323 and flow paths 325may be configured to promote substantially uniform fuel utilization andprevent fuel starvation. A relatively large fuel mass flow may bedirected to the channels 308 along the periphery of the primary flowfield 311 (i.e., along the longest path from the inlet riser channelopening 316 a to the primary flow field 311). This is in contrast toprior art designs (such as shown in FIGS. 2A-2B), in which the highestfuel mass flows tend to be within the middle channels of the flow field(i.e., along the shortest path between the fuel inlet and outlet), whilethe peripheral channels experience the lowest fuel mass flows andhighest fuel utilization, resulting in sub-optimal operationalefficiency and/or irreversible damage to the adjacent fuel cell due tofuel starvation. In embodiments, the fuel distribution flow field 319may be configured to promote uniform fuel utilization across the primaryflow field 311.

The raised features 323 may be configured such that fuel in the flowpaths 325 flow in the second direction, as indicated by arrows 314, overat least a portion of the distribution flow field 319, where the seconddirection deviates by at least 30° (e.g., 45-180°), such as at least 60°(e.g., 90°), from the first direction indicated by arrows 312, beforeentering the primary fuel flow field 311. In embodiments, the flow paths325 in the fuel distribution portion 319 may not be continuous with fuelflow channels 308 in the primary fuel flow field 311, and at least someof the ribs 323, and preferably all of the ribs 323, do not contact theribs 310 in primary flow field 311.

As shown in FIG. 3A, some of the raised features 323 may be ribs 341that are oriented generally perpendicular to ribs 310 of the primaryflow field 311. A line of such ribs (“peripheral blocking ribs”), withgaps between the ribs, may be located proximate to the entrance to theprimary flow field 311 as shown in FIG. 3A. Some of the raised features323 may be ribs 342 that are oriented diagonally (e.g., at an angle ofmore than 30° and less than 60°) relative to the ribs 310 of the primaryflow field 311. Some of the raised features 323 may be ribs 343 thatinclude at least one portion that is generally perpendicular to the ribsof the primary flow field, and at least one portion that is diagonal tothe ribs 310 of the primary flow field 311. As shown in FIG. 3A, theremay be a space 344 between the raised features 323, including theblocking rib 313, and the ribs 310 for fuel flow into channels 308.

FIG. 3B is a schematic cross-sectional illustration of an interconnect 9according to one embodiment, viewed along line A-A in FIG. 3A. FIG. 3Billustrates the anode-side 302 and cathode-side 304 of the interconnect9, including a rib 310 defining the primary flow field over the middleportion 309 of the anode-side 302 of the interconnect 9, as well as oneof the ribs 306 defining the air flow field on the cathode-side 304 ofthe interconnect 9. FIG. 3B also illustrates a number of raised features323 (e.g., ribs, shown in cross-section) defining flow paths 325 in thefuel distribution portion 305 of the anode-side 302 of the interconnect9. As shown in FIG. 3B, the raised features 323 of the fuel distributionportion 305 may have the same height as the ribs 310 in the middleportion 309 of the anode-side 302 of the interconnect 9 to provide auniform topology for the interconnect 9. The blocking rib 313 may alsohave the same height as ribs 310 and raised features 323. Thus, theinterconnect 9 as shown in FIGS. 3A-3B may provide uniform support tothe fuel cell electrolyte, including in the fuel distribution andcollection regions, and may avoid unsupported spans, such as present inthe plenum regions 17 a, 17 b of the interconnect shown in FIGS. 2A-2B.

As is also evident from FIG. 3B, the thickness of the interconnect 9 maybe generally uniform throughout the fuel distribution portion 305, thefuel collection portion 307 and the middle portion 309. The bottomsurfaces of flow paths 325 between features 323 may have the same depthas the bottom surfaces of channels 308 in the primary flow field 311 inarea 309 (i.e., bottoms of 323 and 308 are in the same horizontalplane), expect in area 331. In this embodiment, there is no plenum(i.e., depression in the interconnect surface) extending over the fueldistribution and collection portions. The overall density and densitydistribution of the interconnect 9 may thus be enhanced. The uniformityof the interconnect 9 thickness may be further enhanced providing therespective ribs 310, 306 on the anode-side 302 and cathode-side 304 ofthe interconnect 9 in an offset configuration, as shown in FIGS. 1B and3A of U.S. Published Patent Application No. 2008/0199738 to Perry etal., which is incorporated by reference herein in its entirety.

As shown in FIGS. 3A and 3B, the interconnect 9 may include a fuelcollection portion 307 that is similar to the fuel distribution portion305 described above. The fuel collection portion 307 includes at leastone outlet riser channel opening 316 b for removing excess fuel andreaction products. A raised feature 317 (i.e., blocking rib) is locatedbetween the primary flow field 311 and the outlet riser channel opening316 b. The blocking rib 317 prevents fluid from flowing directly (i.e.,in a straight line path) from the primary flow field 311 to the outletriser channel opening 316 a. In various embodiments, the blocking rib317 is configured such that fluid exiting from the primary flow field311 must flow in a third direction, indicated by arrows 318, beforereaching the outlet riser channel opening 316 b, where the thirddirection deviates by at least 30° (e.g., 45-180°), such as at least 60°(e.g., 90°), from the first direction of fluid flow within the primaryflow field 311. The third direction may not be the same as the seconddirection of fluid flow. For example, as shown in FIG. 3A, the unusedfuel and reaction products that contact against the blocking rib 317 isinitially forced to flow towards the periphery of the interconnect 9,and then must flow approximately 180° in the opposite direction to reachthe outlet opening 316 b. In general, the first direction issubstantially parallel (i.e., within 10°) to a line between the inletopening 316 a and the outlet opening 316 b. The second direction and thethird direction may be a direction that is not parallel to this line,and deviates from this line by more than 30°.

The fuel collection portion 309 of the interconnect 9 may include a fuelcollection flow field 321. The fuel collection flow field 321 mayinclude, in addition to the blocking rib 317, additional raised features(e.g., protrusions) 327, such as ribs, dimples, etc., that define flowpaths 329 between the features. The raised features 327 and flow paths329 may be configured to promote substantially uniform fuel utilizationand prevent fuel starvation. The raised features 327 may be configuredsuch that fuel in the flow paths 329 flow in the third direction, asindicated by arrows 318, over at least a portion of the collection flowfield 321, where the third direction deviates by at least 30° (e.g.,45-180°), such as at least 60° (e.g., 90°), from the first directionindicated by arrows 312. In embodiments, the flow paths 325 in the fuelcollection portion 321 may not be continuous with fuel flow channels 308in the primary fuel flow field 311, and ribs 327 do not contact ribs310. As shown in FIG. 3B, the raised features 327 of the fuel collectionportion 307 may have the same height as the ribs 310 in middle portion309 of the anode-side 302 of the interconnect 9 to provide a uniformtopology for the interconnect 9. The blocking rib 317 may also have thesame height as ribs 310 and raised features 327.

As shown in FIG. 3A, the anode-side 302 of the interconnect 9 mayinclude a depressed area or region 331 surrounding the inlet riserchannel opening 316 a. This depressed area or region 331 may correspondwith an elevated area or region of on the opposite (cathode) side of theinterconnect that is used to seal the riser channel against the adjacentfuel cell and prevent the mixing of fuel and air. A similar depressedarea or region 333 may surround the outlet riser channel opening 316 b.FIG. 3C is a partial cross-section view of the interconnect 9 takenalong line B-B that more clearly illustrates the depressed area 331surrounding the inlet riser channel opening 316 a. The depressed area331 on the anode side 302 of the interconnect 9 may be aligned with andsubstantially correspond to the flat surface 315 a on the opposite(i.e., cathode) side 304 of the interconnect 9. Surface 315 a may have aheight that is the same as that of the cathode side ribs 306 and mayrise above the channels on the cathode side. A fuel hole seal 15 a maybe later deposited on the flat surface 315 a. Since the depressed area331 on the anode side of the interconnect generally matches the flatsurface 315 a on the cathode side of the interconnect, thecross-sectional thickness of the interconnect in the area surroundingopening 316 a may be relatively uniform with the thickness over the restof the interconnect, as shown in FIG. 3C. This, areas of relatively lowdensity surrounding the openings 316 a, 316 b may be avoided, resultingin increased strength and mechanical stability. Also, as shown in FIG.3C, chamfers 334 may be provided around the fuel riser inlet opening 316a to provide even higher-densities around the riser holes. The chamfers334 may be provided on the anode- and/or the cathode-side(s) of theinterconnect 9, and may be provided around the inlet and/or outlet riserchannel openings 316 a, 316 b.

In an alternative embodiment, the anode-side 302 of the interconnect 9may include an elevated region 331 surrounding the inlet riser channelopening 316 a. This elevated region 331 may correspond with a depressedregion on the opposite (cathode) side of the interconnect that is usedto seal the riser channel against the adjacent fuel cell and prevent themixing of fuel and air. A similar elevated area 333 may surround theoutlet riser channel opening 316 b. In general, if either an elevationor a depression is located one side (anode or cathode side) of theinterconnect, then an opposite depression or elevation can be added tothe opposite side of the interconnect in order to maintain as constantinterconnect cross-sectional thickness as possible. The bottoms of allchannels/flow paths may be in the same horizontal plane (except inareas, such as area 331, opposite an elevated area on the other side ofthe interconnect), and the tops of all protrusions/ribs may be in thesame second horizontal plane.

Because the density gradients and lack of support for the electrolyte inprior art systems are primarily driven by the existence of the fuelplenum(s), various embodiments of the invention provide flow fieldfeatures (e.g., ribs, dots or other raised features/protrusions) in theregion(s) previously utilized for the plenum(s). These flow fieldfeatures may be configured to contact the adjacent fuel cell and provideuniform support to the fuel cell electrolyte in the plenum region.Furthermore, the large density gradients resulting from the plenums(i.e., surface depressions) in prior art interconnects may be avoided.

If the area where the fuel is being distributed is also active, thenmore fuel must be delivered to the regions where the fuel distributionis occurring. The distribution regions bring the fuel away from thecenter of the interconnect, and a mechanism is provided by which fuel isdirected to flow away from the center, where openings 316 a, 316 b arelocated. In embodiments, this may be accomplished by the blocking ribsbetween the openings. Through the implementation of the blocking rib(s)and distribution and collection flow fields, the following benefits maybe gained:

(A) Increased cell efficiency through maximizing rib contact in the cellactive area; and

(B) Increased stack yields through improved cell structural supportby 1) eliminating the unsupported span of the fuel plenum, and 2)improved topography due to more uniform density throughout theinterconnect.

FIG. 4 illustrates a portion of an interconnect 9 according to a furtherembodiment. In this embodiment, the fuel distribution flow field 419includes raised features in the form of dots 423 (e.g., pillars or bumpshaving circular, oval or polygonal shape). The dots 423 may support andprovide electrical contact with the electrolyte of the adjacent fuelcell, and together with the blocking rib 313, may provide a flowdistribution that promotes uniform fuel utilization. Similar featuresmay be provided in the fuel collection flow field. The interconnect 9may otherwise be identical to the interconnect 9 illustrated in FIGS.3A-3C. The dots 423 may have a random distribution or may be formed in apattern (e.g., having a higher density closer to opening 316 a and alower density proximate to the periphery of the flow field).

FIG. 5 illustrates a portion of an interconnect 9 according to a furtherembodiment. In this embodiment, the fuel distribution flow field 519includes raised features that include both ribs 523 and dots 524. Theribs 523 and dots 524 may support and provide electrical contact withthe electrolyte of the adjacent fuel cell, and together with theblocking rib 313, may provide a flow distribution that promotes uniformfuel utilization. Similar features may be provided in the fuelcollection flow field. The interconnect 9 may otherwise be identical tothe interconnect 9 illustrated in FIGS. 3A-3C. As shown in FIG. 5, theribs 523 may be oriented substantially perpendicular to the ribs 310 ofthe primary flow field 311, although in other embodiments, some or allof the ribs 523 may be diagonal and/or a combination of diagonal andperpendicular ribs. Also, the dots 524 may have a random distribution ormay be formed in a pattern (e.g., having a higher density closer toopening 316 a and/or primary flow field 311, and a lower densityproximate to the periphery of the distribution flow field 519).

FIGS. 6A-6B illustrate a portion of an interconnect 9 according to afurther embodiment. FIG. 6A shows a portion of the interconnect 9 inplan view, and FIG. 6B schematically illustrates a cross-section of theinterconnect 9 viewed along line C-C. In this embodiment, the fueldistribution flow field 619 includes regions 601, 603, 605 of varyingdepth to provide varying degrees of flow restriction to the fuelentering through inlet riser channel opening 316 a. As shown in FIG. 6B,a first region 601 may have the greatest depth to provide minimal flowrestriction. This first region 601 may direct fuel from opening 316 a tochannels 308 on the periphery of the primary flow field 311. A secondregion 603 may have an intermediate depth to provide an intermediateflow restriction. The second region 603 may direct fuel from opening 316a to channels 308 between the periphery and middle portions of theprimary flow field 311. A third region 605 may have the shallowest depthto provide the greatest flow restriction. The third region 605 maydirect fuel from the opening 316 a to channels 308 in the middle portionof the primary flow field 311. The variation in depth between thedifferent regions 601, 603, 605, which results in a varying degree offlow restriction between the regions, may provide uniform distributionof fuel to the primary flow field 311 and promote uniform fuelutilization. Similar features may be provided in the fuel collectionflow field. The interconnect 9 may otherwise be identical to theinterconnect 9 illustrated in FIGS. 3A-3C.

It will be understood that the distribution flow field 619 may have moreor less than three regions of varying depth. Furthermore, although theregions 601, 603, 605 shown in FIGS. 6A-B have a stepped contour, thevariation in depth between the regions may be graduated (e.g., over acurved or sloped surface).

Also, while FIG. 6A illustrates a blocking rib 313 that forces the fuelfrom opening 316 a to flow in a lateral direction through thedistribution flow field 619 before entering the primary flow field 311,it will be understood that the distribution flow field 619 may providesufficiently uniform distribution of fuel to the primary fuel flow field311 without requiring a separate blocking rib 313. One or more of theregions 601, 603, 605 may be generally “L”-shaped having a first segmentthat extends from the fuel opening 316 a in a direction that isgenerally perpendicular to the direction of ribs 310 and then a secondsegment that extends towards the primary flow field 311 in a directionthat is generally parallel to the direction of ribs 310.

Optionally, raised features (e.g., ribs, dots, etc.) may be provided inone or more of regions 601, 603, and 605.

FIGS. 7A-7B illustrate another embodiment of an interconnect 9 for afuel cell stack. For an interconnect 9 formed by powder metallurgy, itis desirable to achieve the highest densities in regions most sensitiveto crossover leaks. Powder metallurgy techniques for increasing densitylocally are not precise and have proven to only increase variations indensity and topography within the interconnect. The design features inthe embodiment of FIGS. 7A-7B may help achieve optimal density andtopography without the use of these techniques.

As discussed above, prior art interconnects, such as shown in FIGS.2A-2B, employ a design for distributing fuel throughout the cell thattends to further increase density variation, and unfortunately tends toproduce lower density in the region most sensitive to crossover leaks.The fuel plenums (e.g., 17 a, 17 b in FIGS. 2A-2B) distribute the fuelfrom the fuel inlet opening 16 a out to the flow field at the inlet, andcollect the excess fuel and reactant products at the outlet and deliverthem to the fuel outlet opening 16 b. The plenums are relatively wideand deep depressions that drive the density to be the highest in thepart due to small cross-sectional areas in this region. As discussedabove, the compaction process tends to “bottom-out” when this highdensity region approaches 100% dense. The stroke of the compaction pressis limited due to the high density region being incapable of beingcompacted further. As a result, the density achieved elsewhere in thepart is somewhat pre-determined at a lower level by the extent of thecompaction stroke. Complicating things, this region of high density isinterrupted in the center by flat, elevated regions (e.g., regions belowfuel hole seals 15 a, 15 b in FIG. 2A; also region 315 a in FIG. 3C)surrounding the fuel holes on the cathode side of the interconnect.These flat areas are provided for locating a seal (e.g., fuel hole seal15 a, 15 b) that separates the fuel from the air. This flat elevatedsurface creates a region of large cross-sectional thickness, especiallywhere the elevated surface extends out beyond the fuel plenum on theopposite (fuel) side of the interconnect. A lower than average densityis often observed in this region, proving to be the source of failuremodes in stacks utilizing interconnects of the prior art. This is theregion is where the fuel and the air are in closest proximity, and thusis most sensitive to cross-over leaks.

Powder metallurgy manufacturing techniques have been developed toaddress some of these density concerns. The local density is determinedin large part by the ratio of the thickness of the powder beforecompaction to the thickness of the final part. If the final partthickness is assumed to be fixed, the density can be adjusted byincreasing or decreasing the amount of powder that is compressed intothat thickness. The delivery of the powder to the compaction press canbe modified to take powder away from one region and add powder to otherregions to facilitate higher or lower density. However, forinterconnects used in fuel cell stacks, these techniques have proveninsufficient to achieve the required density without negativerepercussions.

Various embodiments achieve improvements in interconnect density andtopography by providing an interconnect having more uniformcross-sectional areas throughout the part. This may be implemented byintroducing design features that would be recognized as mildlycounter-productive from a functional perspective but achieve muchstronger benefits as a result of enabling optimized density andtopography.

FIG. 7A is a perspective view of a portion of the anode side 702 of aninterconnect 9 according to one embodiment. On the anode side 702,increased density uniformity may be achieved through increasing thedensity in the region 731 corresponding to the flat elevated region 715a surrounding the riser channel opening 716 a on the cathode side 704 ofthe interconnect 9, as illustrated in FIG. 7B. It is noted that region731 is the same feature as 331 in FIGS. 3A, 3C, 4, 5 and 6A but in aplenum 717 rather than in a flow field. The fuel is delivered on theanode side 702 from the fuel opening 716 a and is distributed throughoutthe rest of the flowfield 711 by means of the plenum 717 a. The plenum717 a comprises a depressed region in the surface of the interconnectthat includes a central region 731 around opening 716 a having a greaterdepth than the peripheral regions 732 of the plenum 717 a. In general, afuel plenum 717 a that is deeper in a central region 731 than in theperipheral regions 732 would be recognized as counter-productive towardachieving optimal fuel distribution. It is, however, a relatively weakinfluence. By implementing a deeper fuel plenum 717 a in the region 731directly underneath the elevated surface 715 a surrounding the fuelopening 716 a on the cathode side 704, a small sacrifice in achievingfuel distribution (which can be counteracted by other means) canachieve, with great precision, a significant increase in interconnectdensity where it is needed the most. An identical configuration may beprovided in the fuel collection portion of the interconnect 9.

FIG. 7B is a perspective view of the cathode side 704 of theinterconnect 9 shown in FIG. 7A. On the cathode side 704, increaseddensity uniformity may be achieved through an increase incross-sectional thickness in a raised portion or relief 734 thatcorresponds to the fuel plenum 717 a on the anode side 702 of theinterconnect (i.e., a plenum relief). Air enters into the cathodeflowfield 750 through the channels 758 defined by ribs 760 that extendto the first edge 701 of the interconnect 9, and it travels down thechannels 758 until it exits the interconnect 9 at the second edge (notshown) (i.e., for an interconnect that is internally manifolded for fueland externally manifolded for air). The plenum relief 734 will producesome flow restriction as the air enters the cathode flow field 750(i.e., the air side channel 758 depth is smaller over the plenums on thefuel side of the interconnect than over the primary flow field 711 onthe fuel side of the interconnect). In general, features that restrictthe flow of air are counter-productive. However, restrictions that actover a relatively short distance are a weak influence. By introducing aplenum relief 734 that elevates the bottom surface of the airflowchannels 758 in the region directly over the anode fuel plenum 717 a, asmall sacrifice in air flow restriction (which can be counteracted byother means) can achieve, with great precision, a design that delays thepoint at which the compaction press bottoms-out, increasing the strokelength of the press, and thereby increasing the density throughout therest of the part. An identical design may be employed at the oppositeend of the interconnect 9, with a plenum relief 734 corresponding to thefuel collection plenum on the anode side of the interconnect. In variousembodiments, the plenum relief 734 may be located over substantially theentire plenum 717 a on the anode side of the interconnect, which as usedherein may be at least about 75% (e.g., 85-90%), including 95-100% ofthe area of the plenum 717 a.

The fuel plenum relief 734 on the cathode-side 704 of the interconnect 9and the depression 731 surrounding the fuel opening on the anode-side702 of the interconnect 9 are both features which would generally berecognized as counter-productive. However, in the context of design formanufacturing, these features may produce benefits that outweigh thecosts. These features may allow for more uniform contact between theinterconnect and the electrolyte while maintaining required densitylevels and a more uniform interconnect thickness in different regions.Conversely, these features may allow for increased density withoutincreasing the topography variation. These designs may provide improvedyields and increased reliability, and may be advantageously used insingle press powder metallurgy techniques.

Further embodiments provide design improvements on the interconnect andendplates. In one embodiment, the channel 8 depth on the fuel (anode)side of the IC (see FIGS. 1-2B) is greater adjacent to the fuel inletand/or outlet riser opening(s) 16 a, 16 b than distal from the fuelriser opening to account for cross sectional thickness difference due tothe existence of region 315 a (i.e., a flat surface around riseropenings 16 a, 16 b on which seals 15 a, 15 b sit) on the correspondingair (cathode) side of the interconnect 9. In other words, channel 8depth on the anode side is greater over region 315 a on the cathode sidethan over ribs 760 on the cathode side of the interconnect 9).

In an additional embodiment, the height of ribs 10 on at least one ofthe fuel and/or air side of the interconnect 9 in a portion adjacent tothe fuel inlet and/or outlet riser opening(s) 16 a, 16 b is less thanthe height of the same ribs distal from the fuel riser opening.Specifically, tips on the ribs around the opening(s) 16 a, 16 b on bothair and fuel sides of the interconnect 9 are omitted or removed tocounter high topography as a result of density non-uniformity caused bythe region 315 a (i.e., flat surface on which seals 15 a, 15 b sit).These embodiment interconnect configurations may be used alone or incombination with each other to help achieve better density uniformitywhile decreasing the resultant topography variation. Either or both ofthese embodiment interconnect configurations may also be used incombination with any of the embodiments previously described in thisapplication.

FIG. 10A is a perspective view of a portion of the anode side of aninterconnect 9 according to one embodiment. The fuel is delivered on theanode side from the fuel opening 16 a and is distributed throughout therest of the primary flowfield 811 by means of the plenum 817 a. Anidentical configuration may be provided in the fuel collection portionof the interconnect 9 (not shown in FIG. 10A). The plenum 817 acomprises a depressed region in the surface of the interconnect that mayextend at least about 60% around the circumference of the riser channelopening 16 a, such as at least about 90%, and in some embodimentsentirely around the circumference of the riser channel opening 16 a. Ina non-limiting example shown in FIG. 10A, the plenum 817 may include afirst portion 831 having a greater depth than one or more secondportion(s) 832 of the plenum 817 a. As shown in FIG. 10A, the first(deeper) portion 831 of the plenum 817 a may encompass a roughlysemi-circular or semi-oval region that curves around the riser channelopening 16 a adjacent to the primary flowfield 811, and the secondportion 832 may be adjacent to the periphery of the riser channelopening 16 a. In other embodiments, the first (deeper) portion 831 mayalso extend around the periphery of the riser channel opening 16 a, asshown in FIGS. 7A-B. In other embodiments, the entire plenum 817 a mayhave a uniform depth.

As shown in FIG. 10A, an additional depressed region 818 may extend intothe primary flowfield 811 on the fuel side of the interconnect 9. Theadditional depressed region 818 may be contiguous with the plenum 817 a,and may have the same depth as at least a portion of the plenum (e.g.,the first portion 831 of the plenum 817 a, as shown in FIG. 10A). Thedepressed region 818 may encompass a roughly semi-circular or semi-ovalportion of the flowfield 811 which curves around the opening 16 a andfirst portion 831 of the plenum 817. The shape of the depressed region818 may correspond with at least a portion of the elevated surface 315 asurrounding the riser channel opening 16 a on the opposing (air/cathode)side of the interconnect. The depressed region 818 of the flowfield 811results in a greater channel 8 depth in the depressed region 818 than inthe remainder of the primary flowfield 811. Thus, the depth of the flowchannels 8 adjacent to the fuel inlet and/or outlet riser opening(s) 16a, 16 b (i.e., within depressed region 818 over surface 315 a) may begreater than the depth of those same channels 8 or different channelsdistal from the riser opening(s) 16 a, 16 b (i.e., outside of depressedregion 818 and not over surface 315 a on the cathode side of theinterconnect).

This is illustrated schematically in FIG. 10B, which is across-sectional view of an interconnect 9 taken along line D-D in FIG.10A. The channels 8 in the depressed region 818 have a first depth, X₁,that is greater than the depth X₂ of the channels 8 in the rest of theflowfield 811.

The configuration of FIGS. 10A-B may allow more uniform interconnectdensity and reduced topography variation on the rib 10 tips around theopenings 16 a, 16 b. Thus, the regions 818, 817 of relatively deeperdepression may be located only in the region that requires increaseddensification without affecting the functionality of the fuel cellstack. The regions 818, 817 may cover an insignificantly small region ofthe interconnect 9 that will not affect flow characteristics due to fuelcell stack operation at low Reynolds' numbers. This design modificationspecifically targets an area that is functionally sensitive to the fuelcell stack operation to increase interconnet density and avoid fuelpermeating through the flat region 315 a to the seal surfaces.

Embodiments may also include chamfers 33 around the fuel riseropening(s) 16 a, 16 b to provide even higher-densities around the riserholes, as shown in FIG. 10A. The chamfer 33 may provide a lowerthickness region around the riser opening 16 a than in surroundingareas. The chamfer 33 can be provided on the anode- and/or thecathode-side(s) of the interconnect 9.

FIGS. 11A-B are partial perspective views of an air (cathode) side of aninterconnect 9 according to a further embodiment in which the height ofribs 10 on at least one of the fuel and/or air side of the IC in aregion 919 adjacent to the fuel inlet and/or outlet riser opening(s) 16a, 16 b is less than the height of the same ribs and other ribs distalfrom the fuel riser opening outside of the region 919. Specifically,tips on the ribs 10 around the opening(s) 16 a, 16 b on both air andfuel sides of the IC may be omitted or removed to counter hightopography as a result of density non-uniformity caused by the elevatedregion 315 a surrounding the riser openings 16 a, 16 b. This isillustrated schematically in FIG. 11C, which is a cross-sectional viewof an interconnect 9 taken along line E-E in FIG. 11A. The ribs 10 inregion 919 have a first height Y₁ that is less than the height Y₂ of theribs 10 outside of the region 919 (which the bottoms of the channels 8between the ribs have the same depth/are located in the same plane).

As stated earlier, over-densification and compaction methods used toavoid over-densification both cause higher topography variation,especially around regions with high density variation such as the regionincluding and surrounding the flat regions 315 a. These topographyvariations typically manifest themselves in the form of higher rib tipson both the air and fuel sides. In this embodiment, the height of theportions of the rib 10 tips on one or both of the air and the fuel sidesof the interconnect 9 in a region 919 adjacent to the opening(s) 16 a,16 b is reduced compared to the height of the ribs outside region 919 inorder to compensate for the added height during the compaction process.Region 919 may correspond to the same area as the depression 817 on thefuel side (see FIG. 10A) or it may cover a differently sized area.

In one non-limiting configuration, the magnitude of the rib heightreduction may be largest closest to the D-Flat region 932, and may getprogressively smaller the farther away from the flat region 315 a (e.g.,starting at the flat region 315 a under seal 15 a and then varying tothe left and to right in FIG. 11A). The height reduction may bedetermined empirically through monitoring the heights during compactionand making an adjustment in the model for the tool. The resultantinterconnect may therefore be flat with minimal topography variation.Alternatively or in addition, the ribs 10 may have a tapered height inregion 919 (i.e., increasing in direction away from the edge 920 of theIC and/or from opening 16 a) versus a constant rib height outside region919.

While solid oxide fuel cell interconnects and electrolytes are describedabove in various embodiments, embodiments can include any other fuelcell interconnects, such as molten carbonate, phosphoric acid or PEMfuel cell interconnects, or any other shaped metal or metal alloy orcompacted metal powder or ceramic objects not associated with fuel cellsystems.

The foregoing method descriptions are provided merely as illustrativeexamples and are not intended to require or imply that the steps of thevarious embodiments must be performed in the order presented. As will beappreciated by one of skill in the art the order of steps in theforegoing embodiments may be performed in any order. Words such as“thereafter,” “then,” “next,” etc. are not necessarily intended to limitthe order of the steps; these words may be used to guide the readerthrough the description of the methods. Further, any reference to claimelements in the singular, for example, using the articles “a,” “an” or“the” is not to be construed as limiting the element to the singular.

Further, any step or component of any embodiment described herein can beused in any other embodiment.

The preceding description of the disclosed aspects is provided to enableany person skilled in the art to make or use the present invention.Various modifications to these aspects will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other aspects without departing from the scope of theinvention. Thus, the present invention is not intended to be limited tothe aspects shown herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

What is claimed is:
 1. A method of operating a fuel cell stackcomprising fuel cells separated by interconnects, the method comprising:flowing a fuel from a fuel inlet opening in the interconnects to a fueloutlet opening in the interconnects through fuel channels locatedbetween fuel ribs in a primary flow field on a first side of theinterconnects, wherein a larger fuel mass flow is directed to the fuelchannels extending along a periphery of the primary flow field than tofuel channels extending through a middle of the primary flow field; andflowing air on a second side of the interconnects.
 2. The method ofclaim 1, wherein the second side of the interconnects comprises airchannels that extend from a first outer edge of the interconnects to asecond opposing outer edge of the interconnects.
 3. The method of claim2, wherein the interconnects are externally manifolded for air.
 4. Themethod of claim 2, wherein each of the interconnects further comprise adepressed region surrounding the fuel inlet opening that correspondswith an elevated region on the second side of the interconnects.
 5. Themethod of claim 2, wherein each of the interconnects further comprisesan elevated region surrounding the fuel inlet opening that correspondswith a depressed region on the second side of the interconnects.
 6. Themethod of claim 1, wherein each of the interconnects comprises 94 to 96weight percent chromium and 4 to 6 weight percent iron.
 7. The method ofclaim 1, wherein the fuel flow path from the fuel inlet opening to thefuel outlet opening through the fuel channels extending along theperiphery of the primary flow field is longer than the fuel flow pathfrom the fuel inlet opening to the fuel outlet opening through the fuelchannels extending through the middle of the primary flow field.
 8. Themethod of claim 7, wherein a substantially uniform fuel utilizationoccurs across the primary flow field due to a difference in the fuelmass flow and the fuel flow path length through the fuel channelsextending along the periphery of the primary flow field and the fuelchannels extending through the middle of the primary flow field.
 9. Themethod of claim 8, wherein fuel starvation is prevented across theprimary flow field due to the difference in the fuel mass flow and thefuel flow path length through the fuel channels extending along theperiphery of the primary flow field and the fuel channels extendingthrough the middle of the primary flow field.
 10. The method of claim 1,further comprising generating electric current from the fuel cell stack.11. The method of claim 1, wherein the fuel cells comprise solid oxidefuel cells.